The present invention relates to a composite film. More particularly, the present invention relates to a composite film whose surface is coated with polyimide membrane(s), which is excellent in heat resistance.
Films made of polyester resins, especially polyethylene terephthalate resins or polyethylene naphthalte resins, and films made of polyphenylene sulfide resins are widely used in various industrial fields because of the good processabilities, excellent mechanical properties, chemical properties, and the like.
However, the above-mentioned films made of polyester resins and polyphenylene sulfide resins have a drawback in that their glass transition points are low and so their heat resistances are poor for industrial uses at high temperatures, so that they cannot be used as the parts subjected to high temperatures.
In view of this, various composite films have been proposed, in which membranes made of materials which compensate the drawback of the resins are formed on the surfaces of the films made of the resins. For example, Japanese Laid-open Patent Application (Kokai) No. 57-167256 discloses a process for producing a heat resistant resin film by immersing a fluororesin film in a solution of polyamic acid in N-methyl-2-pyrrolidone to coat the film surfaces with the polyamic acid resin, and by heating the resultant to dry. However, to convert the polyamic acid into polyimide resin, dehydration and imidation reaction are necessary, so that heat treatment at a temperature as high as 300xc2x0 C. is necessary.
If the polyamic acid is applied to a film made of a polyester resin or polyphenylene sulfide resin, which has a low melting point, the substrate film per se is melted, so that this method cannot be employed. On the other hand, since ordinary polyimides are insoluble in solvents, thin membranes of such polyimides cannot be formed on film surfaces.
An object of the present invention is to provide a composite film which is excellent in heat resistance and in adhesiveness of the surface layer.
The present inventors intensively studied to discover that by applying a solution of a solvent-soluble polyimide containing a specific compound as an acid component or diamine component and by drying the solution to form a polyimide membrane, a composite film having excellent heat resistance can be obtained and the polyimide membrane has excellent adhesiveness with the substrate film, thereby completing the present invention.
That is, the present invention provides a composite film comprising a substrate film and polyimide membrane(s) formed on at least one surface of said substrate film, which polyimide membrane(s) is(are) prepared by coating said surface(s) with a solution of a solvent-soluble polyimide whose main chain is formed by polycondensation of one or more tetracarboxylic dianhydrides and one or more diamines and by drying said solution, said one or more tetracarboxylic dianhydrides including bicyclo(2,2,2)oct-7-ene-2,3,5,6-tetracarboxylic dianhydride as at least a part thereof, and/or said one or more diamines including at least one of 3,5-diaminobenzoic acid and a diaminosiloxane derivative as at least a part thereof.
Since polyimide membrane(s) which is(are) excellent in heat resistance, chemical resistance and insulation performance is(are) formed on the surface(s) of the substrate film, the composite film according to the present invention is excellent in heat resistance, chemical resistance and insulation performance. Further, the adhesiveness between the substrate film and the polyimide membrane is excellent. Therefore, the composite film according to the present invention may be applied for various uses for which heat resistance, chemical resistance and/or insulation performance are demanded. The present invention has excellent advantageous effects in that polyimide in the form of membrane can directly be formed on the substrate films with low melting points, so that the advantageous properties of both of the substrate film and the polyimide may be synergistically exerted, that the production is easy and that the production cost is low.
As mentioned above, the composite film according to the present invention comprises a substrate film and polyimide membrane(s) formed on at least one surface of the substrate film. As the substrate film, films made of thermoplastic resins on which polyimide membranes could not be formed thereon hitherto because of the low heat resistances may be advantageously employed. Examples of such a thermoplastic resin include polyesters such as polyethylene terephthalates and polyethylene naphthalates, and polyphenylene sulfides. The thickness of the substrate film is not restricted at all and is usually about 10 to 200 xcexcm. As the substrate film, commercially available various films may be employed.
The composite film according to the present invention is obtained by applying a solution of a solvent-soluble polyimide whose main chain is formed by polycondensation of one or more tetracarboxylic dianhydrides and one or more diamines on at least one surface of the above-mentioned substrate film, and by drying the solution so as to form polyimide membrane(s) on the surface(s).
The polyimide used in the present invention is solvent-soluble. As the solvent, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), N-methylpyrrolidone (NMP), tetramethylurea, sulfolane or the like may be employed. Preferably, DMF or NMP is employed. In the present specification, the term xe2x80x9csolvent solublexe2x80x9d means that the polyimide is dissolved in N-methyl-2-pyrrolidone (NMP) to a concentration of not less than 5% by weight, preferably not less than 10% by weight.
The tetracarboxylic dianhydride(s) used for forming the main chain of the molecule of the solvent-soluble polyimide is(are) not restricted. Examples of the tetracarboxylic dianhydrides include aromatic acid dianhydrides such as biphenyltetracarboxylic dianhydride, benzophenonetetracarboxylic dianhydride, bis(dicarboxyphenyl)propane dianhydride, bis(dicarboxyphenyl)sulfone dianhydride, bis(dicarboxyphenyl)ether dianhydride, thiophenetetracarboxylic dianhydride, pyromellitic dianhydride and naphthalenecarboxylic dianhydride; and acid dianhydrides such as 1,2,3,4-butanetetracarboxylic dianhydride, bicyclo(2,2,2)-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride and 4,4xe2x80x2-{2,2,2-trifluoro-1-(trifluoromethyl)ethylidene}bis(1,2-benzenedicarboxylic dianhydride). These may be used individually or in combination.
Examples of the diamines used for forming the main chain of the molecule of the solvent-soluble polyimide include 4,4xe2x80x2-diaminodiphenylpropane, 4,4xe2x80x2-diaminodiphenylmethane, 3,3xe2x80x2-diaminodiphenylmethane, 4,4xe2x80x2-diaminodiphenyl sulfide, 4,4xe2x80x2-diaminodiphenyl sulfone, 3,3xe2x80x2-diaminodiphenyl sulfone, 3,4xe2x80x2-diaminodiphenyl sulfone, 4,4xe2x80x2-diaminodiphenyl ether, 3,4xe2x80x2-diaminodiphenyl ether, 4,4xe2x80x2-diaminobenzophenone, 3,3xe2x80x2-diaminobenzophenone, 2,2xe2x80x2-bis(4-aminophenyl)propane, benzidine, 3,3xe2x80x2-diaminobiphenyl, 2,6-diaminopyridine, bis{4-(4-aminophenoxy)phenyl}sulfone, bis{4-(3-aminophenoxy)phenyl}sulfone, bis{4-(4-aminophenoxy)phenyl}ether, bis{4-(3-aminophenoxy)phenyl}ether, 2,2xe2x80x2-bis{4-(4-aminophenoxy)phenyl}propane, 2,2xe2x80x2-bis{4-(3-aminophenoxy)phenyl}propane, 4,4xe2x80x2-bis(4-aminophenoxy)biphenyl, 2,2xe2x80x2-bis{4-(3-aminophenoxy)phenyl}hexafluoropropane, 1,5-diaminonaphthalene, 2,2xe2x80x2-bis{4-(4-aminophenoxy)phenyl}hexafluoropropane, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 3,5-diaminobenzoic acid, diaminosiloxane derivatives, 1,4-benzenediamine, 1,3-benzenediamine, 6-methyl-1,3-benzenediamine, 4,4xe2x80x2-diamino-3,3xe2x80x2-dimethyl-1,1xe2x80x2biphenyl, 4,4xe2x80x2-diamino-3,3xe2x80x2-hydrox-1,1xe2x80x2biphenyl, 4,4xe2x80x2-diamino-3,3xe2x80x2-dimethox-1,1xe2x80x2biphenyl, 4,4xe2x80x2-oxybis(benzeneamine), 3,4xe2x80x2-oxybis(benzeneamine), 4,4xe2x80x2-methylenebis(benzeneamine) and 3,3xe2x80x2-carboxybis(benzeneamine), although the diamines are not restricted to these. These diamine components may be used individually or in combination.
To promote the adhesiveness of the polyimide to the substrate film, the polyimide used in the present invention contains, among the above-mentioned tetracarboxylic dianhydrides, bicyclo(2,2,2)oct-7-ene-2,3,5,6-tetracarboxylic dianhydride as a tetracarboxylic dianhydride, and contains at least one selected from the group consisting of 3,5-diaminobenzoic acid and diaminosiloxane derivatives as diamine(s). To attain sufficient adhesiveness, the content of bicyclo(2,2,2)oct-7-ene-2,3,5,6-tetracarboxylic dianhydride in the total tetracarboxylic dianhydrides is preferably 20 to 50 mol %, more preferably about 30 to 50 mol %, and the content of the total of the above-mentioned 3,5-diaminobenzoic acid and the diaminosiloxane derivative in the total diamines is preferably 10 to 65 mol %, more preferably about 35 to 60 mol %.
As the diaminosiloxane derivative, those having amine values (the value obtained by dividing the molecular weight of the compound with the number of amino groups) of about 200 to 1000 are preferred, and bis(3-amino-(C1-C6)alkyl-polydimethylsiloxanes are especially preferred. The diaminosiloxane derivatives having the above-mentioned amine value may easily be obtained by removing low boiling components from the commercially available diaminopolysiloxane derivatives (usually, a mixture of polysiloxanes with different polymerization degrees).
To promote the adhesiveness with the substrate film, the polyimide preferably contains as diamine component(s), in addition to the above-mentioned diaminosiloxane derivative having an amine value of 200 to 1000, at least one diamine selected from the group consisting of 3,4xe2x80x2-diaminodiphenyl ether, 1,4-bis(3-aminophenoxy)benzene, bis-{4(3-aminophenoxy)phenyl}sulfone and 3,5-diaminobenzoic acid. By containing the diamine(s), the adhesiveness with the substrate film is especially excellent. Further, since these diamines are stably and inexpensively available industrially, the polyimide is especially suited for industrial production. In this case, the content of the diaminosiloxane derivative in the polyimide is preferably about 15 to 60% by weight, especially 20 to 50% by weight, and the content of the total of at least one diamine selected from the group consisting of 3,4xe2x80x2-diaminodiphenyl ether, 1,4-bis(3-aminophenoxy)benzene, bis-{4(3-aminophenoxy)phenyl}sulfone and 3,5-diaminobenzoic acid in the polyimide is preferably about 10 to 50% by weight, especially about 13 to 40% by weight.
The polyimide employed in the present invention preferably has a weight average molecular weight measured by gel permeation chromatography method (GPC method using a UV detector) in terms of polystyrene of 15,000 to 100,000, more preferably about 20,000 to 70,000. If the molecular weight is within this range, a composite film having excellent adhesiveness between the polyimide membrane and the substrate film, and excellent heat resistance, chemical resistance and insulation performance is obtained.
The polyimide in the composition according to the present invention may be produced by direct imidation reaction between the above-mentioned tetracarboxylic dianhydride(s) and the above-mentioned diamine(s). In the productions of the conventional polyimide molded articles, polyamic acids are used. The polyamic acids are easily decomposed at room temperature, so that the storage stabilities are poor. Further, to convert the polyamic acid into polyimide, it is necessary to subject the polyamic acid to a heat treatment at 250 to 350xc2x0 C. so as to carry out imidation reaction. In contrast, the polyimide used in the present invention is directly produced by the imidation reaction between the tetracarboxylic dianhydride(s) and diamine(s), and not through a polyamic acid, so that the polyimide in the present invention is largely different from the conventional polyimide molded articles in this respect.
The direct imidation reaction between the tetracarboxylic dianhydride(s) and the diamine(s) may be carried out using a catalytic system utilizing the following equilibrium reaction between a lactone, base and water.
{lactone}+{base}+{water}={acid}+{base}xe2x88x92
A polyimide solution may be obtained by using the {acid}+{base}xe2x88x92 system as a catalyst and heating the reaction mixture at 140-180xc2x0 C. The water produced by the imidation reaction is eliminated from the reaction system by azeotropic distillation with toluene which is a reaction solvent. When the imidation in the reaction system is completed, {acid}+{base}xe2x88x92 is converted to the lactone and the base, and they lose the catalytic activity and are removed from the reaction system. The polyimide solution produced by this process can be applied to the substrate film as it is as a polyimide solution with high purity because the above-mentioned catalytic substances are not contained in the polyimide solution after the reaction.
Examples of the reaction solvent which may be used in the above-mentioned imidation reaction include, in addition to the above-mentioned toluene, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, sulfolane, tetramethylurea and the like.
As the lactone, xcex3-valerolactone is preferred. As the base, pyridine and/or methylmorpholine is(are) preferred.
Crotonic acid may be used in place of lactone.
The mixing ratio (acid/diamine) between the tetracarboxylic dianhydride(s) and the diamine(s) subjected to the imidation reaction is preferably about 1.05 to 0.95 in terms of molar ratio. Further, the concentration of the lactone is preferably about 5 to 30 mol % based on the concentration of the acid dianhydrides, and the concentration of the base is preferably about 100 to 200 mol % based on the lactone, at the initiation of the reaction. The reaction time is not restricted and varies depending on the molecular weight of the polyimide to be produced and the like, and usually about 2 to 10 hours. It is preferred to carry out the reaction under stirring in nitrogen atmosphere.
It should be noted that the production process per se of the polyimide using the binary catalytic system comprising the lactone and the base is known, and described in, for example, U.S. Pat. No. 5,502,143.
By carrying out the above-described imidation reaction sequentially in two steps using different acid dianhydrides and/or different diamines, polyimide block copolymers can be produced. By the conventional process for producing polyimide through polyamic acid, only random copolymers can be produced as copolymers. Since polyimide block copolymers can be produced selecting arbitrary acids and/or diamines, desired properties or functions such as adhesiveness, dimensional stability, low dielectric constant and the like can be given to the polyimide. In the composite film of the present invention, such a polyimide copolymer may also preferably be employed.
A preferred process for producing the polyimide block copolymers include the process wherein a polyimide oligomer is produced using the acid catalyst generated by the above-described lactone and the base, and using either one of the diamine component or the tetracarboxylic dianhydride in excess, and then the diamine and/or the tetracarboxylic dianhydride is(are) added (the molar ratio of the total aromatic diamines to the total tetracarboxylic dianhydrides is 1.05 to 0.95), thereby carrying out two-step polycondensation.
Since the polyimide produced by the method described above is in the form of a solution dissolved in a solvent, the solution may be applied as it is, or after being diluted with toluene, dioxane, dioxolane, anisole or a polar solvent mentioned above, to one or both surfaces of the substrate film. Since the polyimide solution to be applied is a solution of the polyimide after completion of the imidation reaction, unlike the production of the conventional polyimide molded articles through the polyamic acid, it is not necessary to carry out imidation reaction by heating after the application of the solution. Further, since the solution is a solution of the polyimide (not a solution of polyamic acid), the storage stability is good and it is not decomposed upon contact with water, which are advantageous.
Before application of the above-mentioned polyimide solution, the surface(s) of the substrate film is(are) cleaned by washing with water or with a solvent such as acetone or methanol and then drying the solvent. If the surface(s) of the substrate film is(are) clean, this step may be omitted.
The cleaned surface(s) of the substrate film is(are) coated with the above-mentioned polyimide solution and the thickness of the polyimide solution on the film surface(s) is made uniform with a device such as an applicator.
The substrate film whose surface is uniformly coated with the polyimide solution is placed in a drier and the solvent of the polyimide solution is removed at a temperature at which the substrate film is not deformed, that is, at 80xc2x0 C. to 200xc2x0 C., preferably at 90xc2x0 C. to 150xc2x0 C. when the substrate film is a polyester film or a polyphenylene sulfide film.
In cases where polyimide membranes are formed on both surfaces of the substrate film, the above-mentioned step of application of the polyimide and the step of drying are respectively carried out for both the front and back surfaces, thereby the polyimide membranes can be formed on both surfaces of the substrate film.
As for the method of application of the polyimide solution to the film surface, in addition to the method using an applicator, continuous methods for producing composite films, including screen printing method, gravure coating method and spray coating method, which are known in the art, may also be employed.
The thickness of the polyimide membrane after drying is not restricted, and usually and preferably about 0.1 to 30 xcexcm, especially about 0.2 to 10 xcexcm.
The polyimide membranes thus formed, especially polyimide block copolymer membranes, typically have the following physical properties. By selecting the components for forming the polyimide block copolymer, polyimide block copolymers having physical properties which are different from those mentioned below can also be produced.
1) Thermal Properties
Glass Transition Point: 180xc2x0 C. to 350xc2x0 C.
Thermal Decomposition Initiation Temperature: 400xc2x0 C. to 550xc2x0 C.
2) Electrical Properties
Volume Resistivity: Not less than 1017 ohms
Dielectric Constant: 2.5 to 3.5
3) Mechanical Properties
Tensile Strength: 10 to 25 kgf/mm2 
Tensile Elongation: 20 to 200%
Tensile Elasticity: 200 to 350 kf/m2 
Water Absorption: 1.0 to 2.0%
Uses of the composite film according to the present invention, exploiting the above-mentioned properties will now be described.
Exploiting the improvement in the thermal properties, the composite film can be used as a heat resistant composite film. In cases where the substrate film is a polyethylene naphthalate film, although the withstand temperature in mechanical uses is 160xc2x0 C., the withstand temperature of the composite film whose both surfaces are coated with the polyimide membranes formed by applying the polyimide solution to a thickness of 3 xcexcm was improved to 210xc2x0 C.
As for the electrical properties, the volume resistivity was improved from 10xc3x971017 ohmxc2x7cm of the substrate film to 45xc3x971017 ohmxc2x7cm of the composite film.
Therefore, the composite film according to the present invention can be applied to various uses such as wall-paper, surface-protecting films of various articles, insulation films, circuit boards and resistors, which exploit the heat resistance, chemical resistance and/or insulation performance.